1. Field of the Invention
The present invention relates to a fuel cell system. More particularly, the present invention relates to a fuel cell system and a method for operating the same on shutdown and startup.
2. Description of the Related Art
Fuel cells are used as electricity generating systems for generating electrical energy by an oxidation reaction of a fuel and a reduction reaction of an oxidizing agent. Such fuel cells may be classified into polymer electrolyte membrane fuel cells and direct oxidation membrane fuel cells according to the type of fuel they use.
Polymer electrolyte membrane fuel cells generally have superior output characteristics, lower operating temperatures, and faster starting and response characteristics as compared to direct oxidation membrane fuel cells. Polymer electrolyte membrane fuel cell have been widely used as power sources for vehicles, distributed power sources for houses and public buildings, and compact power sources for electronic devices.
A fuel cell system employing a polymer electrolyte membrane fuel cell generally includes a fuel cell stack, a reformer, a fuel supplier, and an oxidizing agent supplier. The fuel supplier generally includes a fuel tank and a fuel pump, and supplies fuel to the reformer. The reformer generates hydrogen gas from the fuel and supplies the hydrogen gas to the stack. The stack generates electrical energy by an electrochemical reaction between the hydrogen gas from the reformer and an oxidizing agent.
However, in fuel cell systems employing a polymer electrolyte membrane fuel cell according to the prior art, hydrogen peroxide (H2O2) may be generated from hydrogen gas remaining in the stack after the system is stopped. This can dehydrate the electrolyte membrane of a membrane-electrode assembly (MEA) and deteriorate its performance. This can cause a reduction in the conductivity of the electrolyte. Therefore, system performance cannot recover quickly when the system is restarted.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.